1. Field of the Invention
The present invention relates to a method and an apparatus which permit accurate measurement of a distribution of surface temperature of an object made of a plurality of different materials the emissivity values of which are not known.
2. Discussion of Related Art
It is sometimes necessary to accurately measure a distribution of temperature, for example, a distribution of a surface temperature of an article placed within a firing or heating furnace, or a distribution of a surface temperature of a heat-generating body. There has been proposed a surface-temperature distribution measuring apparatus which uses an image sensor operable to obtain two images of an object body with respective radiations of different wavelengths selected from an optical energy or light emitted from the object body. This measuring apparatus is arranged to obtain a ratio of radiant intensity values at each pair of corresponding local portions of the obtained two images, and measure the surface temperature of the object body, while utilizing the principle of measurement by a dichroic thermometer. JP-B2-6844 discloses an example of such a surface-temperature distribution measuring apparatus. The apparatus disclosed in this publication is adapted to calculate a distribution of the surface temperature of the object body, on the basis of the ratio of the actual radiant intensity values corresponding to the two different wavelengths, and according to a predetermined equation based on a known relationship between the radiant intensity ratio and the surface temperature. According to this apparatus, the calculation is possible even where the emissivity on the surface of the object body is unknown.
The measuring apparatus disclosed in JP-B2-7-6844 uses an image receiver in the form of a television camera, to detect the radiant intensity values corresponding to the three primary colors RGB (e.g., red color of 590 nm, green color of 530 nm and blue color of 460 nm) of light from the object body, and obtain a plurality of sets of radiant intensity ratios of two colors of the three primary colors, for example, of R and G. The measuring apparatus is arranged to convert the obtained radiant intensity ratios into the surface temperature of the object body according to a predetermined theoretical curve with a compensating function, and display a distribution of the surface temperature.
The conventional surface-temperature distribution measuring apparatus described above uses the selected two colors of the three primary colors of light of the object body. However, the radiant intensity ratio of the selected two colors cannot be considered to be a ratio of the two radiations having the predetermined wavelengths, so that the principle of measurement of the conventional apparatus does not fully match the principle of measurement by the dichotic thermometer, namely, does not fully meet a prerequisite that the dependency of the emissivity value on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the conventional measuring apparatus suffers from a large amount of error included in the obtained surface temperature distribution.
On the other hand, JP-A-7-301569 discloses a method of obtaining a distribution of surface temperature according to dichroism using infrared radiations. This method employs an infrared-radiation camera for detecting the radiant intensity, two filters which permit transmission of respective two radiations having respective different wavelengths, and a mirror the angle of which is changed to permit an incident radiation to be incident upon a selected one of the two filters. According to the present method, the temperature is obtained on the basis of the radiant intensity ratio of obtained images of the object body, and the obtained temperature is displayed for each picture element of the images. Since the wavelengths of the radiations used are close to each other, it is considered that a difference in the emissivity is accordingly reduced, making it possible to reduce the amount of error included in the obtained temperature distribution. However, JP-A-7-301569 does not contain any description on a relationship between the characteristics of the two filters, and a technique that permits measurement of a temperature distribution with a sufficiently high degree of accuracy.
The present invention was made in view of the background art discussed above. It is a first object of the present invention to provide a method which permits accurate measurement of a distribution of surface temperature of an object body. A second object of the invention is to provide an apparatus suitable for practicing the method.
The first object may be achieved according to a first aspect of this invention, which provides a method of measuring a surface temperature of an object body, by calculating a temperature of the object body at each picture element of its image on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective radiations having respective first and second wavelengths and selected from a light emitted from a surface of the object body, the method comprising:
a first wavelength-selecting step of selecting the radiation having the first wavelength from the light emitted from the surface of the object body, by using a first filter which permits transmission therethrough a radiation having the first wavelength which is selected according to a radiant intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature; and
a second wavelength-selecting step of selecting the radiation having the second wavelength from the light emitted from the surface of the object body, by using a second filter which permits transmission therethrough a radiation having the second wavelength which is selected within the high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than ⅓ of the first wavelength and which is not smaller than a sum of a half width of the first wavelength and a half width of the second wavelength.
The second object indicated above may be achieved according to a second aspect of the present invention, which provides an apparatus for measuring a surface temperature of an object body, by calculating a temperature of the object body at each picture element of its image on the basis of a radiant intensity ratio at each pair of corresponding picture elements of a first and a second image which are obtained with respective radiations having respective first and second wavelengths and selected from a light emitted from a surface of the object body, the apparatus comprising:
a first filter for selecting the radiation having the first wavelength from the light emitted from the surface of the object body, the first filter permitting transmission therethrough a radiation having the first wavelength which is selected according to a radiant intensity curve corresponding to a wavelength of a black body at a lower limit of a range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than a radiant intensity at a normal room temperature; and
a second filter for selecting the radiation having the second wavelength from the light emitted from the surface of the object body, the second filter permitting transmission therethrough a radiation having the second wavelength which is selected within the high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than ⅓ of the first wavelength and which is not smaller than a sum of a half width of the first wavelength and a half width of the second wavelength.
In the method and apparatus of the invention as described above, the temperature of the object body at each picture element of its image is calculated on the basis of the radiant intensity ratio at each pair of corresponding picture elements of the first and second images and obtained with the respective radiations of the first and second wavelengths selected from the light emitted from the surface of the object body. Thus, the distribution of the surface temperature of the object body is measured on the basis of the temperature Tij at each picture element. To select the radiation having the first wavelength from the light emitted from the surface of the object body, the present invention uses the first filter which permits transmission therethrough a radiation having the first wavelength which is selected according to a radiant intensity curve corresponding to the wavelength of a black body at the lower limit of the range of the temperature to be measured, and which is within a high radiant intensity range in which the radiant intensity is higher than the radiant intensity at a normal room temperature. The present invention further uses the second filter which permits transmission therethrough a radiation having the second wavelength which is selected within the above-indicated high radiant intensity range, such that the second wavelength is different from the first wavelength by a predetermined difference which is not larger than ⅓ of the first wavelength and which is not smaller than a sum of the half width of the first wavelength and the half width of the second wavelength. Accordingly, optical signals having sufficiently high radiation intensities can be obtained, leading to an accordingly high S/N ratio of the apparatus. In addition, the first and second wavelengths and are close to each other, so that the principle of measurement according to the present invention fully matches the principle of measurement by a dichotic thermometer, namely, fully meets a prerequisite that the dependency of the emissivity on the wavelength can be ignored for two radiations the wavelengths of which are close to each other, leading to approximation xcex51=xcex52. Thus, the present measuring apparatus permits highly accurate measurement of the temperature distribution.
In the method and apparatus of the present invention, the first and second filters are preferably arranged such that the first filter permits transmission therethrough the radiation having the half width which is not larger than {fraction (1/20)} of the first wavelength, while the second filter permits transmission therethrough the radiation having the half width which is not larger than {fraction (1/20)} of the first wavelength. According to this arrangement, the radiations having the first and second wavelengths and are considered to exhibit a sufficiently high degree of monochromatism. Therefore, the present invention meets the prerequisite for the principle of measurement by a dichroic thermometer, resulting in an improved accuracy of measurement of the temperature distribution.
The first and second filters used in the method and apparatus of the invention are preferably arranged such that the first and second filters have transmittance values whose difference is not higher than 30%. This arrangement assures high sensitivity and S/N ratio, even for one of the two radiations of the first and second wavelengths which has a lower luminance value, permitting accurate measurement of the temperature distribution.